pg(s) 151-159 Gypsum,
a highly soluble rock, is readily dissolved to form karst
features identical to those associated with limestones and
dolomites. Investigations in Blaine County, in northwestern
Oklahoma, evaluated potential problems that subsidence due to
gypsum karst may pose for the proposed Watonga Wind-Power
Project, a wind-turbine project just east of Watonga.
Catastrophic collapse of a wind turbine is clearly
unacceptable, and minor settlement could also be a risk.
Differential settlement by even 3 cm across a 15-m-wide turbine
foundation could lead to the turbine tilting out of tolerance,
requiring remedial repairs. Gypsum beds of the Permian Blaine
Formation underlie all parts of the Project Area, at depths
ranging from 10 to 45 m below ground level. The Blaine
Formation here is about 29 m thick: it consists of four gypsum
beds, each 0.6 to 4 m thick, interbedded mainly with red-brown
shales. The Blaine is overlain by the Permian Dog Creek Shale
and by unconsolidated Quaternary sands, clays, and gravels that
may obscure karst features. Field studies, aerial-photo
analysis, and literature review show that there is no evidence
of gypsum karst in the Project Area. Although lacking direct
evidence of karst in or near the Project Area, we recognize
there is some potential for subsidence due to dissolution of
shallow gypsum. Additional mitigation of this risk can be
achieved by placing wind turbines at sites where the gypsum
beds are deepest: we believe that where gypsum is 25 m below
ground level, or deeper, the risk related to gypsum karst is
low. Placing turbines at sites where gypsum beds are less than
25 m deep would pose a medium or high risk. To minimize this
risk, a map was prepared showing areas of low, medium, and high
risk, related to potential gypsum karst.
13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 151 Introduction This study examines the surface and subsurface geology northwestern Oklahoma (Figure 1). The study area embraces nearly 400 km 2 2 Watonga wind turbines would be constructed. The study focuses on the thickness, distribution, structure, and depth of gypsum beds in the Permian Blaine Formation that susceptible to being partially or totally dissolved by Abstract form karst features identical to those associated with limestones and dolomites. Investigations in Blaine County, in northwestern Oklahoma, evaluated potential problems that subsidence due to gypsum karst may collapse of a wind turbine is clearly unacceptable, and minor settlement could also be a risk. Differential foundation could lead to the turbine tilting out of tolerance, requiring remedial repairs. 45 m below ground level. The Blaine Formation here is about 29 m thick: it consists of four gypsum beds, each shales. The Blaine is overlain by the Permian Dog Creek Shale and by unconsolidated Quaternary sands, clays, and gravels that may obscure karst features. Field studies, Although lacking direct evidence of karst in or near for subsidence due to dissolution of shallow gypsum. Additional mitigation of this risk can be achieved by placing wind turbines at sites where the gypsum beds are deepest: we believe that where gypsum is 25 m below ground level, or deeper, the risk related to gypsum karst is low. Placing turbines at sites where gypsum beds are less than 25 m deep would pose a medium or high risk. To minimize this risk, a map was prepared showing areas of low, medium, and high risk, related to potential gypsum karst. GYPSUM KARST AND POTENTIAL RISK IN SITING WIND Kenneth S. Johnson Oklahoma Geological Survey, 1321 Greenbriar Dr., Norman, OK 73072, USA, email@example.com William J. Bangsund Barr Engineering Co., 4700 W. 77 Street, Minneapolis, MN 55435, USA, BBangsund@barr.com Neal A. Hines ENVIRON International, Austin, TX, firstname.lastname@example.org Figure 1. Map of western Oklahoma showing location of Watonga Wind-Power Project and the windenergy-resource potential at 50 m above ground level.
NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 152 groundwater, and to developing karst features such as caves, sinkholes, and underground water courses that commonly are also found in limestones and dolomites dip gently to the southwest where they underlie all parts karst in western Oklahoma, the distribution and depth of the various gypsum beds is important in the siting of wind turbines because of the potential for collapse due to subsurface bedrock dissolution. A wind turbine located above a sinkhole, cave, or other karst feature could become unstable if there is any settlement of the ground. The depth to the top of the uppermost gypsum in the Blaine Formation ranges from 10 to 45 m, and in most of where the depth is 25 m or more, such a site would pose a low risk for problems related to gypsum karst, because if there is karst in these Blaine gypsum beds below that depth it is unlikely that subsidence would reach up to and impact the land surface. We believe that karst in the Blaine gypsums at shallower depths would pose a higher risk for a wind turbine. generated energy (Figure 1). According to the Oklahoma Department of Commerce (personal correspondence, operating in western Oklahoma, with a generating 900 megawatts. Geologic Setting Outcrops of Permian rocks in the Watonga area include gypsum and shale beds of the Blaine Formation, overlain in turn, overlain by Quaternary terrace deposits and alluvium. The Blaine Formation is about 29 m thick in the Watonga area, and individual beds of white gypsum range from 0.6 to 4 m thick (Fay and others, 1962; Fay and several thin beds of dolomite. The thickest gypsum formation. The Blaine Formation dips gently to the south The Dog Creek Shale generally is 50 to 58 m thick in Blaine County (Fay and others, 1962; Fay, 1964), but the to 15 m of shale remains. The Dog Creek is principally several thin beds of gypsum (0.5 to 2 m thick) in the Above the Dog Creek Shale are Quaternary terrace deposits and alluvium deposited by the North Canadian River. that generally are 10 to 20 m thick. All of the proposed wind turbines will be sited directly upon these Quaternary deposits where they overlie the Dog Creek Shale. Methods of Study A comprehensive review of the geologic literature to determine if karst features have been reported in Blaine County and nearby parts of Oklahoma was carried out. possible karst features. Nearby outcrops of gypsum were Figure 2. Geologic map of the Watonga WindPower Project Area in Blaine County (Fay, 2010). Blaine Formation gypsum beds dip gently to the south and southwest (the strike and dip symbols) beneath the Dog Creek Shale and Terrace Deposits that mantle most of the Project Area.
13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 153 categories map (discussed below) to help minimize the possibility that gypsum karst will have an adverse Literature Review A State geologic map was prepared by Miser (1954), followed by detailed maps of the surface geology of Blaine County and the Watonga area by Fay and others (1962) and Fay (2010). These later maps show the outcrop area of gypsum beds in the Blaine Formation, as well as outcrops of the overlying Dog Creek Shale and 2). In addition, Fay (1964) discussed the stratigraphy and character of the Blaine Formation and associated strata throughout northwestern Oklahoma. These three studies by Fay were most valuable for understanding the geology beds elsewhere in western Oklahoma by Myers and caves in the Blaine Formation of western Oklahoma have been examined, mapped, and described by John Oklahoma City: these reports have been released in Oklahoma Underground which is a serial publication Additionally, electrical logs (also called geophysical logs) of many petroleum test wells drilled within and and this was used to determine the depth to the top of the Figure 3. Exposure of the Shimer Gypsum Bed (white) at top of Blaine Formation and the overlying Dog Creek Shale (redbeds). Exposed in US Gypsum quarry about 20 km north of Watonga. Gypsum here is 4.5 m thick. Figure 4. Cross section showing thickness of gypsum beds of the Permian Blaine Formation in outcrops just east of the Watonga Wind-Power Project Area (after Fay and others, 1962). View looking to the southwest.
NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 154 collapse. The small ephemeral ponds present in some of the depressions appear to result from local runoff (from precipitation) into areas that have no external drainage. Field Examination Field studies have discounted the presence of potential sinkholes in terrace deposits, as suggested in the Area were examined, and no evidence of karst features or voids was found. This observation agrees with the (1964), and with recent discussions with John and Sue Bozeman (personal communication, 2010). The one exception is the Foley sink, described above, that is 6.5 depression in the ground. Electric-Log Study About 400 petroleum tests have been drilled in, and wells were examined by Johnson, but only 20 of them contained data about the Blaine Formation or other rock layers present in the top 100 m of the borehole. Recognition of gypsum beds and associated rock types on electric logs is well established (Alger and Crain, 1966), and the senior author has conducted many studies using various types of well logs to identify, correlate, and map gypsum beds in the subsurfacesome of the studies are in public documents (Johnson, 1967, 1985, On each of the 20 useable electric logs in the study area, individual gypsum beds (and interbedded shale units) northeast. Correlation from outcrops to the electric logs shows that the overall thickness of the Blaine Formation, as well as the thickness of individual units, is quite the elevation of the top of the uppermost gypsum (the on gypsum caves of western Oklahoma, John and Sue Bozeman reported that they are not aware of any caves in communication, 2010). gypsum beds here are quite thin and are interbedded with access to the gypsums. The one feature showing possible evidence of gypsum karst is the Foley sink, that formed in 1957 (Fay, 1958). The sink formed in Quaternary Formation. Originally about 15 m wide and 5 m deep, have been carried out to determine the true cause of the sinkhole. Fay also mentioned several older sinks located about 225 m northeast of Foley sink. A statewide, general assessment of potential karst terrains was preliminary, with only general data discussing the various potentially karstic rocks (limestone, dolomite, gypsum, and salt), and showing the general outcrop area of Blaine gypsums in northwest Oklahoma. Aerial-Photo Study Personnel at Barr Engineering in Minneapolis, MN, November 1990; and April 1979). A number of linear drainage features are evident that extend in directions features are related to sinkholes or voids. depressions are present in the Quaternary terrace deposits that blanket the area. Inasmuch as these terrace deposits consists of many sand dunes that create a hummocky glance, may appear to be related to sinkhole development or collapse structures, but there is no evidence that any of the small depressions have, in fact, resulted from ground
13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 155 karst can pose risks to wind turbines. If gypsum karst subsidence or settlement of the overlying Dog Creek Shale and Quaternary sediments. This could result in tilting of, or damage to, the foundation of a turbine built cm across a turbine foundation that is 15 m wide could cause the turbine to tilt and require remedial repairs. karst features in outcrops in the Watonga area, or in the to further reduce the potential risk of gypsum karst on at sites where the top of the Blaine gypsum beds is at least 25 m below the land surface. Structure-Contour Map and Cross Section plotting the elevation of the top of the Blaine Formation Results of Study As a result of the foregoing studies, it is possible maps to select sites for construction of wind turbines where the risk is low, with regard to possible gypsum karst. Potential Karst Risks Evaporite rocks, mainly gypsum (or anhydrite) and rock salt, are the most soluble of common rocks. They can be dissolved readily to form caves, sinkholes, documented in the Watonga area in the literature, probably because the gypsum beds here are quite thin shales that inhibit groundwater access to the gypsum. Evidence of gypsum karst would include surface and disappearing streams, springs, collapse structures, and while drilling through gypsum beds. None of these features have been found. Figure 5. Electric logs of two wells showing depths (in feet) and log characteristics of gypsum beds in the Blaine Formation in (and near) the Watonga Wind-Power Project.
NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 156 shows the approximate elevation of the uppermost from the elevation of the land surface to determine the in the shallowest and thickest gypsumthe Shimer A cross section (Figure 7) shows subsurface relationships between the Blaine gypsums and the overlying Dog Creek Shale and the Quaternary terrace deposits and based on outcrop information from Fay and others (1962 and Figure 4), and on the structure map on top of the Area where turbines might be constructed, they would be located upon Quaternary terrace deposits or alluvium. Immediately beneath these Quaternary deposits, remnants of the lower Dog Creek Shale are present and would separate the Quaternary deposits from the Shimer barrier that provides added protection against karst by sediments into underlying gypsum beds. Risk Map A schematic cross section shows possible karst conditions in gypsum, related to the depth of gypsum below the land on the outcrop and on electric logs of petroleum tests. The Blaine Formation dips gently and uniformly to the 0.1 to 0.2 degree). This map is most useful because it Figure 6. Structure-contour map showing elevation on top of the Blaine Formation gypsums beneath the Watonga Wind-Project Project Area. Contour interval is 6 m (20 feet). Cross section AB shown on Figure 7. Figure 7. Cross section AB through the Watonga Wind-Power Project Area showing the dip of strata and depths of gypsum beds of the Blaine Formation. Cross section based upon: Figure 6; outcrop data; two borings by Barr Engineering Co. (borings #87 and #76); and electric logs of oil-well tests (including Texaco, Horsley Unit B No. 1, shown in Figure 5). Line of cross section shown on Figure 6.
13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 157 Being a highly soluble rock, gypsum is readily dissolved to form karst features identical to those associated with limestones and dolomites. Settlement of a turbine karst features could lead to tilting of the turbine out of tolerance and require remedial repairs. Therefore, we examined the local geology to evaluate whether gypsum surface (Figure 8). It is reasonable to expect that placing wind turbines at sites where the gypsum beds are 25 m deep, or deeper, would pose a low risk related to gypsum karst subsidence. Sites where the gypsum is more than 6 m, but less than 25 m, below the surface are sites of medium risk, and sites where the gypsum is 6 m, or less, below the surface are sites of high risk. a risk map (Figure 9) was prepared by Barr Engineering of the land surface with the elevation of the Blaine Formation (Figure 6). This established the depth to the top of the conservative. Although there is no evidence of gypsum karst in the Watonga area, it is possible that dissolution At least 25 m of sand, clay, gravel, and shale between development and collapse structures, and turbine sites in those areas would be at low risk. Using these criteria and the risk map, it will be possible to locate any proposed Therefore, most of the area is considered to pose a low risk for problems related to gypsum karst. Summary and Recommendations Figure 8. Schematic cross section showing low-, medium-, and high-risk karst zones, which are related to the depth of possible karstic gypsum in northwestern Oklahoma (modified from Johnson, 2003c). Figure 9. Risk categories at Watonga Wind-Power Project, based upon depth to the Shimer Gypsum at top of the Blaine Formation. Contour lines from Figure 6.
NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 158 Fay RO. 1958. A recent sink hole in central Blaine County, Fay RO. 1964. The Blaine and related formations of northwestern Oklahoma and southern Fay RO. 2010. Preliminary geologic map of the and mineral resources of Blaine County, Bulletin 89. 258 p. Johnson KS. 1967. Stratigraphy of the Permian Blaine Formation and associated strata in southwestern Oklahoma [PhD dissertation]. Champaign (IL): University of Illinois. 247 p. Johnson KS. 1985. Structure contour map and Scale 1:125,000. [Blaine Formation, southwestern Oklahoma]. NSS Blaine gypsumdolomite aquifer, southwestern during Salado time in the Wink area, Winkler County, Texas. In: Love DW, and others, editors. Carlsbad region, New Mexico and west Texas. Annual Field Conference. p. 211. United States, In: Johnson KS, Neal JT, editors. problems in the United States. Oklahoma Blaine Formation and associated strata of western Oklahoma. In: Johnson KS, Neal JT, editors. problems in the United States. Oklahoma The Blaine Formation is about 29 m thick, and consists of four gypsum beds, each 0.6 to 4 m thick. The top of the gypsums is 10 to 45 m below ground level in the Creek Shale and by unconsolidated Quaternary sands, clays, and gravels that may obscure karst features. Field that there is no direct evidence of gypsum karst in the Examination of electric logs of petroleum tests enabled us to prepare a cross section and risk map showing the depth to the top of gypsum, and enables us to determine sites where the risk due to gypsum karst would be low. We believe that those locations where the depth to gypsum the risk would be moderate where the depth to gypsum is at those locations where gypsum is less than 6 m deep. We suggest further evaluation of the potential for gypsum karst in the Watonga area by considering the following actions: 1) examine cores drilled through the Shimer evidence of dissolution in that shallowest and thickest gypsum bed; 2) conduct a survey of landowners within collapse structure is reported by landowners, it should survey of landowners where the Blaine Formation crops gypsum dissolution. References with electrical well logs. In: Rau JL, editor. Second Oklahoma gypsum caves. In: Johnson KS, Neal JT, editors. Evaporite karst and
13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 159 terrains in Oklahoma for avoidance of engineering and environmental problems. In: Johnson KS, Neal JT, editors. Evaporite karst and engineering and environmental problems in Martinez JD, Johnson KS, Neal JT. 1998. Sinkholes in Survey. Scale 1:500,000.
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